NGWS SDK Documentation  

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Compiler Error C3138

'interface' : a 'attribute' interface must inherit from IDispatch, or from an interface that inherits from IDispatch

An interface with the dual or dispinterface attriubtes does not have IDispatch as a direct or indirect base interface.

The following code shows how this error would be generated:

// compile with cl /c
#include <unknwn.h>
#pragma keyword("interface", on)

[ com, custom, uuid("77ac9240-6e9a-11d2-97de-0000f805d73b") ]
interface IMyCustomInterface 
{
   HRESULT mf1(void);
};

[ com, dispinterface, uuid("3536f8a0-6e9a-11d2-97de-0000f805d73b") ]
interface IMyDispInterface : IUnknown
{
   HRESULT mf2(void);
};

[ com, dual, uuid("34e90a10-6e9a-11d2-97de-0000f805d73b") ]
interface IMyDualInterface : IMyCustomInterface
{
   HRESULT mf3(void);
};